Dawe AF SIgnal Generator XT 439

[djrm]

I've been on a bit of a roll fixing test gear and decided to get out my old audio sig-gen. Ive had it for a long time, it came from the local army surplus store (Eley's) in around 1970. it has a manufacturing date of 1961 on it.

I've botched a couple of repairs over its life, a new zener diode and some additional capacitors patched onto the existing ones. Today i've just noticed the waveform is a bit distorted and its not very well calibrated either.

The problem is that I have no information about it. There is a bank of eight locking preset resistors which look a bit tempting to fiddle with ( I should know better )

Its built in a sturdy sealed case and can operate from mains or internal battery. The case is aluminium. A cover screws down over the front with room for the detachable mains cable.   

I'd like to have the confidence of a reading a schematic before diving in. it would be a pain to reverse engineer it. 

Doe anybody have any idea where I could find out about it please?
Kind regards, David.

       

[djrm]

There is a locking pre-set resistor on the front panel, it seems to adjusts the oscillator feedback gain, turning it down a bit has removed the noticeable distortion.

[djrm]

Most of the distortion is now due to the other oscillator used to draw this circle.
   

[Mike Watterson]

Info is sparse
https://www.radiomuseum.org/m/dawei_gb_en_1~year.html

I'd heard of Dawe, but never seen that WD monster you have!

I use a good sound card or external USB box with free desktop software for audio now. Generators, scope, analysers, counters, distortion measuring etc. Depending on DAC and electronics the 3.5mm jack socket on some Android phones and tablets can be better than older gear, or very cheap audio function generators. Might be better than the 'scope, to measure distortion, though a nice scope Some old HP, inc one of their original bits of test gear, had very low distortion audio. Specially selected filament lamp in the feedback loop reduced distortion.

I also have one random function generator/counter (up to 2MHz and not low distortion on the sine wave) and a nearly unusable user interface DDS 0.01 Hz to 30 MHz, but very low distortion.
The 1.5 GHz Rigol spectrum analyser + tracking generator mysteriously starts at 19 kHz. It works as a 0 to -20dB fixed signal gen by setting span to 0Hz (there is a menu for that). A nanoVNA isn't much good as a generator.

[djrm]

Thanks for the link but there is nothing similar there. The oscillator has a bulb or thermistor in a glass envelope for amplitude stability.
I'b been using a universlal T&M system frequency counter that i've just repaired, it has a simple 2MHz sine/triangle/square generator built in - nothing special and it has a little pip on the sinewave crest.
I agree there are some very good free utilities available fro PC and phones. I like to keep the old 'junk' going though.
I haven't seen PC distortion analyser, i'll look for one.
Even though the Nano VNA is very good at what it does it isnt much good at doing anything else.
I was wondering whether to by the similar spectrum analyser until I saw a very cheap handheld radio with spectrum analyser function.
I may have access to one of those Rigol test sets, i'm not sure.

[Mike Watterson]

I have an older Visual Analyser on an old XP in the Workshop
https://www.sillanumsoft.org/

The AC97 motherboard chips may or may not be any good depending on associated HW.
https://www.sillanumsoft.org/download.htm

External USB sound boxes / adaptors vary from studio quality to junk.


My Rigol replaced an old HP141T that died, though it had no tracking generator. There are many variations.
Though I had a 10 GHz plug-in for the HP141T, I decided the 19 kHz to 1.5GHz plus TG was enough
https://www.amazon.de/-/en/gp/product/B00CLWJA38/

An RLB is maybe over €500, some are near €1000. I found a 4 port hybrid coupler that works quite well (in a drawer of junk). One sample port has 50 Ohm sealed on it. Something like that can be bought for under €100. The same drawer has an excellent low power DC switched attenuator rated DC to 3GHz that has 10, 20, 20 and 20 dB. So that will do 0 dB to 70 dB in 10dB steps. The Rigol's TG does 0dB to -20 dB in 1 dB steps, so I can use it either as a static generator or wobbulator (IF) or tracking generator (with power sweep option of 0 to -20dB) 19kHz to 1.5GHz at 0 dB to -90dB with 1 dB steps (50 Ohm). No AF modulation, but I'd hardly ever use that and I have the two old valve signal generators with AM or CW 100kHz to 60MHz.
The Spectrum Analyser part can be 50 or 75 Ohms and using DC block splitter and a €45 sat box for volts and 22 kHz I was able to align a sat dish with four quattros and a multiswitch on 9E, 13, 19E and 28.2E, even though at 1.5GHz max the full range can't be seen. An actual satellite receiver is only any use if you have enough signal for a lock and know which bird it is.

I have also a NanoVNA and it's got a bit of a learning curve and can be misleading. Higher frequencies are done using the generator harmonics! Also it has no RLB (return loss bridge) so is using a hybrid coupler. Handy to check aerials. I found that to cut multiple 1/4 wave lengths of 75 Ohm and 50 Ohm coax the best way to find the initial length was to solder the open end to an SMA and hunt with a pin to short the cable 1/4 wave away at the band of interest. Tables of velocity factors are not exact enough, partly because the randomly bought cable may not be same dielectric. I hope to make up a switch box fed from a crossed yagi with dual feed for 145 MHz so I can select +45, -45, vertical, horizontal, left circular and right circular. The S/H aerial as it came was disassembled with some broken insulators (uses rods insulated from boom) and the two driven elements had interconnected pair of gamma matches. I replaced socket on one gamma match so that the aerial has two feeds, a +45° and -45°. The NanoVNA allows one to adjust the position of the gamma match. I also used the NanoVNA to tune a 70MHz vertical and length of four radials (about 1m each obviously!) based on recycling a Watson 300. It was too short so the top cap was cut off and some white PVC pipe with an end cap extended it.

After much experimenting and calculations I decided that a dual band 6m & 4m colinear/vertical is nearly impossible. The 6m, 2m and 70cm or any combination is more feasible.